Fire alarm with bistable characteristics

ABSTRACT

A FIRE ALARM DEVICE EXHIBITING BISTABLE CHARACTERISTICS, THE DEVICE COMPRISING A DETECTOR ELEMENT WHICH PROVIDES AN ELECTRIC SIGNAL IN RESPONSE TO A PHENOMENON ACCOMPANYING COMBUSTION, AND AN AMPLIFYING DEVICE CONNECTED WITH SAID DETECTOR ELEMENT IN A FEEDBACK CIRCUIT IN A MANNER SUCH THAT THE APPEARANCE OF AN ELECTRIC SIGNAL PRODUCES A CHANGE IN THE CONDITION OF THE AMPLIFYING DEVICE WHICH, IN TURN, PRODUCES A CHANGE IN THE POTENTIAL APPLIED TO THE DETECTOR ELEMENT RESULTING IN THE MAINTENANCE OF THE CHANGE IN CONDITION OF THE AMPLIFYING DEVICE UNTIL SUCH TIME AS A RE-SETTING OPERATION IS PERFORMED ON THE CIRCUIT.

Jan. 26, 1971 A. SCHEIDWEILER 3,559,196

FIRE ALARM 'WI'TH BISTABLE CHARACTERISTICS Filed 1:60.27. 1967 s Sheets-Sheet 1 MEASURING IONIZATION CHAMBER G PROR ART ATTORNEY Jan. 26,1911 A.sHE.DwE|LER' 3,559,196

FIRE ALARM WITH BISTABLE CHARACTERISTICS Filed Dec 2'7, 196'? 5 Sheets-Sheet 2 MEASURING IONIZATIQNY CHAMBER l Fly 3 METER FET 7 nub I I MEASURING 5 lONIZATION CHAMBER INVENTOR AnJnEns c'admfla.

ATTORNEY 26, 9 A. SCHEIDWEILER v 3,559,196

FIRE ALARM WITH BISTABLE CHARACTERISTICS Filed Dec 27, 1967 s Sheets-Sheet 2 METER 2/ 22 16 T 1 1 MEASURING IONIZATION CHAMBER INVEN'I OR Audllens SJwiJwel 'el ATTORN EY United States Patent O 3,559,196 FIRE ALARM WITH BISTABLE CHARACTERISTICS Andreas Scheidweiler, Staefa, Switzerland, assignor to Cerberus AG, Mannedorf, Switzerland, a corporation of Switzerland Filed Dec. 27, 1967, Ser. No. 693,937 Claims priority, application Switzerland, Dec. 29, 1966, 18,751/ 66 Int. Cl. G08b 17/10 US. Cl. 340-237 10 Claims ABSTRACT OF THE DISCLOSURE A fire alarm device exhibiting bistable characteristics, the device comprising a detector element which provides an electric signal in response to a phenomenon accompanying combustion, and an amplifying device connected with said detector element in a feedback circuit in a manner such that the appearance of an electric signal produces a change in the condition of the amplifying device which, in turn, produces a change in the potential applied to the detector element resulting in the maintenance of the change in condition of the amplifying device until such time as a re-setting operation is performed on the circuit.

BACKGROUND OF THE INVENTION This invention relates to fire alarm devices consisting of at least one combustion sensitive element which responds to a phenomenon accompanying combustion by the production of an electric signal, the electric signal being then applied to an electronic circuit including a transistor which responds to the signal.

Combustion sensitive elements which respond to phenomenon accompanying combustion such as the development of smoke, a rise in temperature, the appearance of flames and the like, and which in response to these phenomena develop an electric signal, are known in numerous variations. Thus, smoke-sensitive, light-sensitive and temperature-sensitive elements have already been employed for a long time in fire alarms to produce an electric signal indicative of combustion. To amplify the voltage appearing across the sensitive element in a fire alarm, numerous electronic circuits and components have been employed.

The cold-cathode tube had proved particularly suitable for this amplification purpose, but the cold-cathode tube has since been successfully replaced by the field-effect transistor. Both the field-effect transistor and the coldcathode tube, however, have the advantage of a high input impedance and are preferably employed in conjunction with high-impedance combustion sensitive elements such as the smoke-sensitive ionization chamber. When lowimpedance combustion sensitive elements are employed, it is obviously possible to employ another conventional amplifier element such as, for example, a transistor, a thermionic valve or the like.

By the use of suitable circuits, these amplification components can also be made to function as threshold level detectors, so that a substantial alteration in signal occurs at their output only when the potential at their input exceeds a predetermined value. The output of the transistor, which in the following description is employed generally to symbolize any amplifier element, is connected to an electronic circuit of which the primary pur- 3,559,196 Patented Jan. 26, 1971 pose is to pass on any alarm signal which arises to some remote central position. The electronic circuit may contain elements which serve to supervise the complete operational readiness of the fire alarm. Finally, the fire alarm itself may include an indicator device which indicates the condition of the system, that is, ready for operation, alarm or failure.

In most known fire alarms, the combustion sensitive element, the amplifier device, and the electronic circuit form a signal chain; the electronic circuit thereby usually providing an alarm signal to the remote central position in response to an alarm signal initiated by the combustion sensitive elements so long as, and only so long as, the combustion sensitive element provides the corresponding signal voltage, that is, so long as the amplifier element is retained in its altered condition. For various reasons of fire protection technology, it is, however, a requirement that the alarm, once initiated, must remain in its alarm condition even when the alarm-initiating condition subsequently is effectively or apparently abolished, so that, as a result the combustion sensitive element no longer provides an alarm-initiating voltage. Fire alarms have therefore been proposed in which the electronic circuit includes a bistable circuit element which serves to store the alarm condition until the alarm is actually reset, usually from the remote central position.

Apart from economically unsuitable expensive circuit arrangements, the bistable elements formerly used have primarily comprised controlled rectifiers and electromechanical relays. However, controlled rectifiers have the disadvantage that they may easily be caused to ignite and change over into the alternative stable condition by external interference. If this disadvantage is to be avoided, aged and stabilized circuit elements such as capacitors and Zener diodes must be employed, the addition of which makes the alarm more expensive and moreover reduces the reliability of the system over the period considered.

Accordingly, it is a primary object of the instant invention to provide a fire alarm which overcomes the difficulties and disadvantages associated with prior art devices.

It is another, more specific object of the invention to provide a fire alarm of the kind initially described which affords protection against interference and which is reliable over the period considered.

It is yet another specific object of the invention to provide a fire alarm in which storage of an alarm condition is effected at the least possible expense.

SUMMARY OF THE INVENTION The above objects and other advantages of the subject invention are implemented in the physical manifestation thereof in that there is disclosed a fire alarm device including a detector element which provides an electric signal in response to a phenomenon accompanying combustion, and an amplifying device connected with said detector element in a feedback circuit in a manner such that the appearance of an electric signal produces a change in the condition of the amplifying device which, in turn, produces a change in the potential applied to the detector element resulting in the maintenance of the change in condition of the amplifying device until such time as a re-setting operation is performed on the circuit.

The inventive system consisting of a combustion sensitive element, an amplifying device, and an electronic feedback circuit is, by these means, rendered bistable and accordingly differs in principle even from a further variety of the known fire alarms in which the two combustion sensitive elements are connected in series in a bridge connection with an electronic circuit, the bridge connection and the electronic circuit forming a feedback system having a monostable characteristic. In this known system, as contrasted with the present invention, an alarm signal is given only so long as the bridge circuit is unbalanced, effective or only apparent diminution of the alarm-initiating condition thus returning the bridge into balance and suppressing the alarm signal.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood and still further features and advantages thereof will become readily apparent when reference is given to the following detailed description read in conjunction with the appended drawings in which:

FIG. 1 shows a circuit diagram useful for explaining the prior art;

FIG. 2 is a circuit diagram of one embodiment of a fire alarm device according to the invention;

FIG. 3 is a circuit diagram of a modified form of the arrangement shown in FIG. 2;

FIG. 4 shows another modification of the arrangement described in relation to FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, a known arrangement of a fire alarm is depicted as including a combustion sensitive element 1 consisting of an ionization chamber with a resistance element 2 connected in series with it across a power supply represented only by negative and positive terminals 11 and 12. The ionization chamber 1 is arranged such that the ambient air has almost unhindered access thereto. Because of the physical characteristics of the ionization chamber, its electrical resistance changes upon the entrance of smoke particles resulting in a corresponding change in the voltage appearing across the ionization chamber.

The magnitude of the electric current which flows in the absence of smoke aerosols within the ionization chamber is determined by the strength of the radioactive preparation associated with the chamber, the ionizing radiation therefrom making the air in the chamber conductive. An amplifying device is depicted as a field-effect transistor 4, the grid of which is connected to the ionization chamber 1 in such a manner that the voltage across the chamber controls the current in the field-effect transistor. Accordingly, the voltage across resistor 6 is then a measure of the voltage across the combustion sensitive element 1. An electronic circuit controlled by the amplifier element consists of a controlled rectifier 8a which is connected in series with a meter 9 across the supply. The trigger or control electrode of rectifier 8a is returned to the positive line of the supply by way of a resistor 26 and is connected to the emitter of transistor 4 by means of a Zener diode 25. When the voltage across the resistor 6 exceeds the Zener voltage of diode 25, the rectifier 8 is triggered so that current passes through meter 9. This condition is maintained until the circuit is re-set by removing the supply. The meter 9 serves to indicate the occurrence of an alarm condition.

FIG. 2 shows a first embodiment of an alarm device constructed in accordance with the invention. As in FIG. 1, here again an ionization chamber 1 is connected in series with a resistor 2 which may comprise a second ionization chamber and the junction of these components is connected to the grid of a field-effect transistor 4. The cathode or source electrode of the field-effect transistor is connected to a tapping of a voltage divider formed by a fixed resistor and a variable resistor 6 connected in series across the supply. The field-effect transistor operates as. an amplifier element and as threshold device, the voltage at which the transistor response to an input Signal being adjustable by means of the variable resistor 6. The drain electrode of the field-effect transistor 4 is connected to the negative terminal 11 of the direct-current source by means of a resistor 7, and to the base of a further transistor 8, the emitter of which is connected to negative terminal 11 and the collector of which is returned by way of a resistor 3, connected in series with ionization chamber 1, to the positive terminal 12 of the DC. source. Reduced to its most elementary form, an indicating apparatus at a central station, shown to the right of terminals 11, 12, consists of a meter 9 connected in series with a DC. source 10 between the terminals.

In the rest condition of the circuit arrangement, transistor 8 is cut off. When the ionization chamber 1 responds to the presence of combustion aerosols, its resistance increases so that the voltage between the source and grid or gate electrodes of the field-effect transistor 4 rises to the turn-on value. There then appears across resistor 7 a voltage drop which causes transistor 8 to become conductive. Consequently, there then appears across resistor 3 a voltage applied by way of the voltage divider formed by the series combination of ionization chamber 1 and resistor 2 to the gate electrode of fieldeffect transistor 4, such that transistor 4 is made still more conductive. Accordingly, positive feedback is provided. The effect of this feedback is that the voltage drop across the ionization chamber is almost removed, while the potential appearing at the terminals of the chamber is practically equal to that of supply terminals 11, so that the field-effect transistor 4 remains in the conductive condition until the alarm device is re-set by an intentional resetting of the alarm. This stable condition is maintained even when the condition in the ionization chamber which has given rise to the alarm again falls below the alarm-initiating value, so that the chamber itself yields a signal having a value below the alarm threshold level.

A further substantial advantage results from the switching-off of the voltage across the ionization chamber when the alarm device has responded to an alarm condition. It is generally known that, as a result of the electrical charging of combustion aerosols in the ionization chamber, a separation of the aerosols at the electrodes of the ionization chamber takes place to a greater or lesser extent depending upon the magnitude of the electric field strength set up within the chamber. Such separation deleteriously affects the functional reliability of the fire alarm. However, if the voltage applied across the ionization chamber is removed when the alarm responds, then, since there is substantially no electric field within the chamber, there can be no further separation of aerosols. The fire alarm of the instant invention is therefore suitable for continuous use without making provision for extensive cleaning operations.

Ionization chamber 1 has been considered merely as a resistance. In practice, however, it also has a selfcapacitance which, in FIG. 2, is shown as a capacitor 13. This fact has a positive effect upon the reliability of the feedback system, since the resulting time-constant substantially reduces the probability of the appearance of a false alarm as a result of the effect of external interference in the form of short voltage peaks or impulses. The ionization chamber has an internal resistance of the order of 10 ohms and an internal capacity of some 10 pf., so that a time-constant of approximately 1 second is produced. In addition, the field-effect transistor also possesses a small self-capacitance, so that without additional circuit components the fire-alarm device of FIG. 2 has considerable protection against spurious signals.

As already explained, field-effect transistor 4 and transistor 8 do not pass current in the quiescent or rest condition of the apparatus. Therefore, the rest current and thus the power consumption of the alarm in the normal operational condition is kept small, which is a very important consideration in equipment with several hundred alarm devices per group. This arrangement also provides the additional advantage that the transistors commence to conduct if the ambient temperature rises considerably, so that the fire-alarm also operates as a maximum-temperature alarm. The invention can, however, obviously also be operated with both transistors conductive in the rest condition, or with one transistor conductive and one cut off.

However, even in the conductive condition, the fieldetfect transistor 4 and the transistor 8 require only a small amount of power in comparison with the known controlled rectifier arrangements of FIG. 1. This is particularly important if the alarm is coupled with a device which, for test purposes, applies an alarm-simulating condition to all alarms in a group of alarms simultaneously and supervises the simultaneous response of all the alarms. In this case, because of technical installation consider.- ations and because of the necessity of providing the appropriate amount of power, the alarm current of each individual alarm device in a group must be kept as small as possible.

FIG. 3 shows a combination of an alarm device in accordance with the invention together with a device for the periodic supervision of the operational effectiveness of the alarm device. A plurality of alarm devices can be provided although only one is shown for purposes of explanation. The collector of transistor 8 in each alarm device is connected by means of an individual diode 14 with a common lead 15 which, at the central location, is connected with the negative pole of voltage source 10 by means of the parallel combination of two circuits, one circuit comprising a switch 17 in series with a resistor 18, and the other circuit comprising a resistor 16 in series with a current-indicating meter 19. If switch 17 is briefly actuated, the potential at the low-potential terminal of ionization chamber 1 is increased from a value which is practically equal to the potential of supply lead 12 to a value determined by the voltage divider consisting of resistor 3 and resistor 18. Thus, the voltage applied between the gate and source electrodes of field-effect transistor 4 is increased, so that this transistor passes current and the alarm device is triggered into its stable alarm condition. The diode 14, together with resistor 16, forms an AND gate. Thus, if all the tested alarm devices respond, then all diodes 14 remain blocked and no current flows through the meter 19. If, however, at least one of the alarms is defective, then current flows through resistor 3 of the alarm and through meter 19 which indicates the presence of a fault.

An actual alarm given by an alarm device results in current flowing in the meter 9 connected in supply lead 11. By appropriate choice of the value of resistor 3 in each alarm device, the current flowing in case of an actual alarm can be given a desired value, preferably about twice the rest current. The re-setting of an actuated alarm device is effected by interrupting the supply lead 11 by means of an interrupter 20.

FIG.'4 shows a modification of the circuit arrangement described in FIG. 2. In this case, there is connected in parallel with variable resistor 6 the emitter-collector path of a further transistor 21, the base of which is connected through a resistor 22 with the collector of transistor 8 and through a resistor 23 with supply lead 12. When field-effect transistor 4 and the transistor 8 respond, transistor 21 also becomes conductive and thus practically short-circuits variable resistor 6. The voltage applied between the gate and source electrodes of field-effect transistor 4 is thus additionally increased and brought practically to the full supply voltage. The minimum supply voltage or maintaining voltage necessary to maintain an alarm condition which has arisen in an alarm device is thus given only by the threshold voltage of the field-effect transistor. The arrangement of FIG. 4 is particularly suitable when the supply voltage applied to the alarm device cannot be assumed to be constant, i.e., when the supply leads have a relatively high resistance represented as a resistor 24, or when the source of supply voltage itself is not of sufficiently low impedance.

Although the circuit arrangements of FIGS. 2-4 have been described merely in relation of combustion smoke detectors, in which the combustion sensitive element consists of an ionization chamber, it is obviously also possible for similar circuit arrangements to be constructed using optical, thermal or other sensitive elements to provide the alarm condition. Thus, for example, in an optical alarm device, the sensitive element 1 would consist of a photocell which, by means of the field-effect transistor and the associated electronic circuitry, controls a light source. In this case, feedback is effected optically, light from the light source being allowed to fall upon the photocell. Similar arrangements would be provided for other sensitive elements.

It should now be apparent that the objects set forth at the outset of this specification have been successfully achieved.

What is claimed is:

1. A fire alarm device comprising: combustion detector means for providing an electric signal in response to a phenomenon accompanying combustion; amplifying means responsive to said electric signal for amplifying said electric signal; and feedback circuit means connecting said amplifying means to said combustion detector means, said feedback circuit means including said combustion detector means, said feedback circuit means including means for producing a change in voltage across said combustion detector means upon response of said amplifying means to maintain said response of said amplifying means.

2. A fire alarm device as defined in claim 1, further comprising reset means for resetting said feedback circuit means.

3. A fire alarm device as defined in claim 2, further comprising a power supply, means connecting said combustion detector means across said power supply; wherein said amplifying means comprises a first transistor having a control electrode coupled to said combustion detector means, and first output electrodes; and wherein said feedback circuit means comprises a second transistor having a base electrode coupled with said first output electrodes of said first transistor, and second output electrodes connected in series with a resistor across said power supply; and wherein said means connecting said combustion detector means across said power supply includes said resistor.

4. A fire alarm device as defined in claim 3 further comprising biasing means for said first and second transistors for biasing said first and second transistors into a non-conducting rest condition, said first and second transistors conducting current upon the presence of said electric signal from said combustion detector means.

5. A fire alarm device as defined in claim 4, wherein said combustion detector means comprises a ionization chamber connected in series with a second resistor, said control electrode of said first transistor being connected to the junction of said ionization chamber and said second resistor.

6. A fire alarm device as defined in claim 4, wherein said combustion detector means comprises two series connected ionization chambers, said control electrode of said first transistor being connected to the junction of said two series connected ionization chambers.

7. A fire alarm device as defined in claim 3 wherein said first output electrodes of said first transistor includes an emitter electrode, and variable resistor means connecting said emitter electrode to said power supply.

8. A fire alarm device as defined in claim 3, further including variable resistor means for coupling said first output terminals of said first transistor across said power supply; a third transistor having an emitter-collector path shunting said variable resistor and having a base electrode coupled with said second output electrodes of said second transistor, whereby said third transistor substantially shortcircuits said variable resistor when said first and second transistors respond to said electric signal from said combustion detector means.

9. A fire alarm device as defined in claim 3, testing means at a central station for simulating an alarm condition at said fire alarm device; said second output electrodes of said second transistor of each fire alarm device comprising an emitter electrode and a collector electrode; means including a diode for connecting the collector electrode of each second transistor of each fire alarm device to a common lead, whereby a logic circuit is formed which indi- References Cited UNITED STATES PATENTS 2,759,174 8/1956 Brailsford 340-237 10 DONALD J. YUSKO, Primary Examiner D. MYER, Assistant Examiner U.S. Cl. X.R. 

